Theme cruises are all the rage these days.
But when the RV JOIDES Resolution set sail out of Mobil, Ala., in early June for a 30-day tour in the Gulf of Mexico, it represented a whole different kind of special interest sojourn -- one that's not included in travel agents' vacation brochures.
Rather than indulging in 'round the clock gourmet spreads and free flowing libations synonymous with cruise ships, the select group of passengers on this vessel will be working non-stop to unravel the mysteries of overpressure and fluid flow in the shallow sediments of the deepwater GOM.
"The ship is truly a floating scientific laboratory where a lot of science is done on the ship," said AAPG member Peter Flemings, professor in the department of geosciences at Penn State University and co-chief scientist on the expedition, along with Jan Behrmann from Germany.
"We'll have 28 full-time scientists and 20 full-time staff on this ship, which has built a history on acquiring a lot of core rapidly," Flemings continued. "Our whole job will be to continually look at core and describe grain size, age, paleo-magnetic character, pore fluid chemistry and such."
This venture is being implemented under the auspices of the Integrated Ocean Drilling Program (IODP), an international research program created to explore the history and structure of the earth recorded in the rocks and sediments of the seafloor.
The IODP structure is not just multidisciplinary, but multinational:
- The United States is responsible for providing a riserless drilling vessel.
- Japan will provide and operate a riser vessel.
- A number of European participants will provide mission-specific platforms to venture into environments unsuited for these other boats.
The systems integration contractor for the riserless vessel and related activities is an alliance comprising the Lamont-Doherty Observatory, Columbia University, Texas A&M University and Joint Oceanographic Institutions Inc.
"The intention of Expedition 308 in the Gulf is to try to understand very shallow overpressure -- all our wells will be less than 1,500 feet subsea," Flemings said.
"One of the biggest reasons overpressure is generated is a lot of sediment is deposited very fast and water can't get out, so the water ends up supporting a certain amount of rock," he noted. "Where there's really rapid deposition but there's also a permeable layer -- for example, you take a bunch of mud and bury a permeable sand, then you can set up some interesting flow fields that end up driving some practical problems.
"We'll be studying the overpressure and what it means for how fluid -- especially water -- is flowing through those shallow sediments.
"People have been worrying about how to drill overpressured wells for years," Flemings noted. "When you're drilling in the very shallow sand section and encounter ovepressured sands that are very unconsolidated, the sands can just bubble out of the borehole -- it looks like a witches brew.
"You can lose a well easily."
Areas of Interest
A well-known example of pricey problems caused by shallow water flow is Shell's Ursa location at Mississippi Canyon, where Flemings noted Shell lost a template because of the treacherous environment.
In fact, the GOM expedition will set anchor at the Ursa basin, which is one of two scheduled stops.
"We've used numerical models to explain what we think is happening in the system there, and we've seen evidence of landslides in the seismic data," said expedition scientist Brandon Dugan, assistant professor Department of Earth Sciences at Rice University, and an AAPG member. "Now we're going to test fluid flow models by taking sediment samples to bring back to the lab to measure physical properties like permeability and compare to what's used in the models.
"We'll make direct measurements of fluid pressure, which is hard to get in shales," Dugan said. "We'll be testing some new equipment for that, and this will be one of the first direct tests for theoretical models, which is a new venture for IODP."
Submarine landslides, which are common in the shallow subsurface, also will be under scrutiny by the scientists. Oil and gas operators need to know when and how a slope might fail to avoid placing a rig in the vulnerable location.
The Brazos Trinity Basin due south of Galveston also is on the ship's itinerary.
"We're drilling a number of locations that are of great interest to people studying turbidites," Flemings said. "A lot of them have studied the particular turbidites in this location and how they've formed, but this is the first time we'll have cores and logs through it.
"The simple model is that a turbidite is like a snow avalanche under water, where the sediments ran downhill and were eventually deposited," Flemings noted. "Eventually these piles of sand get buried and become oil reservoirs."
Studying one of these very shallow piles of sand -- in what we call an intra-slope basin -- can help to answer a lot of questions, including:
- How thick are the sands?
- How do they pinch laterally?
- What is the age of the sand relative to the history of sea level?
"Some of the greatest industry interest is in looking at these shallow areas as analogs for much deeper reservoirs," Flemings noted. "By understanding these better, the hope is to use the information to predict in much deeper systems where the good sands are that will eventually be turbidite production."
Preparing for the Passed-Over
Another particularly noteworthy focus of this venture is the evaluation of the subsurface between a depth of 500 feet and as deep as 2,000 feet, which is rarely sampled by operators, according to Flemings.
People who build the pipelines and platforms are interested in the region above this depth, and operators ordinarily just "pass through" on the way to deeper horizons. Besides, the large diameter of the borehole at the shallow section of the well makes it difficult for the operator to run logs or do much of anything.
"We'll provide a large suite of experiments and measurements on that shallow section for everyone," Dugan said. "It will be public domain data.
"The oil and gas industry would love to know how the porosity changes with depth, what the rock types are, what the acoustic, sonic and density logs say.
"Anyone drilling even a shallow well will penetrate these shallow sands and face a number of potential problems," Dugan noted. "Landslides initiate here, and there are also problems with boreholes collapsing and pipelines shearing off.
"We're trying to understand the processes in these rocks that cause not just the loss of wells but a number of other problems."
The oil and gas industry clearly stands to benefit from expeditions such as the current GOM cruise; it also played a key role in making it happen.
"This project would not be possible without significant preliminary support and data provided by the industry," Dugan noted. "For instance, without the input of data and experience from industry who drilled these locations before, we would never have been able to get past the hurdle of the safety and hazard review."